Speed control for a rotary machine

ABSTRACT

A first capacitor is discharged with every pulse of a train of pulses of a frequency proportional to the actual speed of a rotary machine under control and is charged up during intervals between these pulses. The voltage of the first capacitor provides a signal to a threshold circuit which operates to provide a second signal whenever the voltage of the first capacitor exceeds a predetermined voltage which may be set in order to set the speed at which the machine is to be controlled. During the presence of the second signal a second capacitor is charged, beginning at a predetermined voltage, preferably zero. The regulating magnitude for the machine to be controlled is derived from the voltage of the second capacitor either directly or indirectly through a controller circuit and a power stage. The circuit is particularly useful for control of the speed of machines that run slowly or can conveniently provide only relatively few speed-indicating pulses per revolution.

This invention concerns a speed control for a rotary machine, such as anelectric motor, a turbine, or an internal combustion engine, in whiche.g. a tachogenerator driven by the machine provides a signal of afrequency proportional to rotary speed, which is then processed so as toprovide an output which can be used for speeding up or slowing down themachine whenever it slightly deviates from the desired speed.

From DE-PS No. 1 254 170, an electrical speed control system is knownfor controlling the speed of a turbine. A tachogenerator is coupled tothe turbine. Its output frequency is multiplied and then modulates in aring modulator an alternating voltage of a prescribed frequency. Thismodulated wave, after passage through a low-pass filter isdifferentiated to obtain a train of pulses which periodically activate atransistor that discharges a capacitor that continuously receives asubstantially constant charging current from a current source. If thedifferentiated pulses follow each other closely, the charge voltage ofthe capacitor reaches only low values. When pulses follow each other atgreater intervals, the charge voltage of the condenser reaches highervalues. This charge voltage is sensed with a threshold switch, theoutput voltage of which controls the speed regulation process, forexample, by controlling a valve that opens or closes the steam supply tothe turbine.

In this known system, there is accordingly involved a two-pointregulation system, which is unsatisfactory for many purposes thatrequire speed regulation. Furthermore, particular precautions must betaken in order to prevent the machine on running up to speed fromrunning right through the regulation range and consequently continuingto increase in speed out of control. For this purpose, the known systemrequires a special protection circuit.

THE INVENTION

It is an object of the present invention to provide a speed control thatavoids the disadvantages of the known speed-control system and,particularly, a speed control capable of keeping the controlled machinerunning very precisely at the controlled speed, without involving riskor loss of control in start-up.

According to the invention, a capacitor that is discharged in everypulse of a pulse train of a frequency proportional to the speed and ischarged between pulses furnishes the value of the charge voltage as asignal to a threshold circuit which is used to control the charge of asecond capacitor, the charging of the second capacitor beginning, eachtime, from a predetermined value, when the voltage of the firstcapacitor rises past the predetermined value, the latter beingpreferably capable of being set so as to provide a speed setting.

The second capacitor charges either directly or indirectly through acontroller or the like, determines the value of the regulating currentor other magnitude used for varying the speed of the motor. To start thecharging of the second capacitor each time from a predetermined value,the threshold circuit is caused to produce a signal equivalent to theonset of a normal output signal responding to the voltage of the firstcapacitor exceeding the predetermined voltage value, in order to providea brief pulse for discharging the second capacitor.

Preferably a differentating circuit is connected between the outputs ofthe threshold circuit and the normalizing circuit used for dischargingthe second capacitor, in order to discharge the latter either upon theappearance either of a normal output signal of the threshold circuit orof an equivalent signal provided for the purpose in the manner alreadymentioned.

There are several ways of providing the aforesaid equivalent signal. Itmay be derived from the same pulses that provide for discharging thefirst capacitor, either through a separate amplifier or an electronicswitch, or through a separate differentiating circuit and it may bedesirable to utilize a decoupling resistor where two inputs are providedto the same input of the threshold circuit, the latter being preferablyin the form of a comparator such as may be constituted by means of anoperational amplifier.

Because of the repeated normalization of the second capacitor before thestart of each charging thereof, there is no danger of obtaining a falsecontrol allowing further increase of speed when the speed is already toohigh.

A speed control system according to the invention is particularly usefulfor control of the rotary speed of a motor from which only a relativelylow pulse frequency proportional to motor speed is available, as in thecase of a slowly-running motor for directly driving a turntable used forplaying or recording sound records and, per rotor likewise, afast-running motor which provides, for example, only one control pulseper rotor revolution.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of illustrative examples withreference to the annexed drawings, in which:

FIG. 1 is a circuit diagram of a preferred embodiment of a speed controlcircuit according to the invention;

FIG. 2 is a graphical presentation of wave forms and the like on acommon time scale for explanation of the circuit of FIG. 1;

FIG. 3 is a circuit diagram of a second embodiment of the invention;

FIG. 4 is a graphical presentation of wave forms on a common time scalefor explanation of the circuit of FIG. 3;

FIG. 5 is a graph for further explanation of the manner of operation ofthe circuit of FIG. 1; and

FIG. 6 is a circuit diagram of a third embodiment of the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 shows a first and preferred embodiment of the invention. As thereshown, a tachogenerator 10 has a permanently magnetic rotor 11 which isdriven by the motor 12 under speed control, and a stator winding 13which produces a sinusoidal output voltage 14. The motor 12 is normallya brushless dc motor, but it is to be understood that the invention isalso applicable to other rotary motors and engines, for example, forspeed control of a turbine or of an internal combustion engine.

The signals 14 are supplied to a pulse-shaping stage 15 that includes abandpass filter. The output of stage 15 produces, at the output of thecoupling condensor 46 of the following stage, sharp needlelike pulses 16of a frequency corresponding to that of the signals 14. These pulses arethen supplied to a frequency-to-voltage convertor 17, at the output 18of which an analog voltage is obtained that is a function of the speedof the motor 12. This voltage is then supplied to an amplifyingcontroller 19 with a proportional-integral PI behavior orcharacteristic. At its output 20 the controller 19 provides controlsignals for a current amplifier 21, the output of which provides thecurrent for the motor 12.

In this way the current in the motor 12 is so controlled that the motorspeed is held constant within very narrow limits.

In the illustrated embodiment, the motor 12 runs at 200 rpm and thetachogenerator 10 accordingly provides a frequency of 200 Hz. For thisreason, the stage 15 is so designed that it favors the transmission offrequencies between 100 and 1000 Hz. For starting up, very lowfrequencies must naturally also be allowed to pass. A resistor 25 (100ohms) and a capacitor 26 (1.5 μf) operate as a low-pass filter, while acapacitor 27 (1 μf) and a resistor 28 (1.5 k ohms) operate as ahigh-pass filter. An operational amplifier 29 serves for pulse shaping.At its output 30, a rectangular voltage wave 31 is obtained. The output30 is connected through a resistor 32 of high resistance with thenon-inverting input of the operational amplifier 29, which input is alsoconnected through a resistor 33 with an output of the tachogenerator 10,the other output of which is connected, through the series combinationof resistor 25 and capacitor 27, with the inverting input of theoperational amplifier.

The terminal of the tachogenerator connected to the resistor 33 is alsoconnected through the parallel combination of a capacitor 36 (15 μf) andthe resistor 37 (1 k ohms) with the negative voltage bus 38 thatoperates effectively as ground or reference potential. A positivevoltage bus 41 is supplied by a regulated voltage of, for example, +5volts by a voltage regulator 39 connected to a voltage source 40. Aresistor 44 is connected between the positive bus 41 and a connectionpoint or node which is connected through a resistor 28 with theinverting input of the operational amplifier 29 and via the resistor 33with the noninverting input of the operational amplifier. The stage 15thus converts the sinusoidal signal 14 into a rectangular signal 31 andpreferentially amplifies the frequencies between 100 and 1000 Hz. Thesame stage 15 is also used in the embodiment of FIG. 3 and for thatreason the presentation is not repeated there.

A differentiating network consisting of a capacitor 46 and a resistor 47converts the rectangular signals of the output 30 of the stage 15 intothe needlelike pulses 16 that appear at the connection point 48. Onlythe positive-going pulses are utilized.

The converter 17 contains an operational amplifier 51 that serves as thethreshold circuit and is therefore connected as a comparator. Itsnoninverting input is designated 52, its inverting input 53, and itsoutput 54. A resistor 55 of high resistance is connected between theoutput 54 and the noninverting input 52. A voltage divider consisting oftwo equal resistances 56 and 57 is connected between the positive andnegative busses 41 and 38, and has its tap 58 connected through aresistor 59 with the noninverting input 52 of the operational amplifier.

A first capacitor 62 of, for example, 30 nF is connected between theinverting input 53 of the operational amplifier and the negative bus 38.An adjustable resistor 63 of, for example, 200 k ohms, determines therate of charge of the capacitor 62 and serves to set the desired speedat which the motor is to be controlled. The first capacitor 62 isassociated with a discharge circuit branch in the form of an npntransistor 64 that has its collector connected to the inverting input 53of the operational amplifier and its emitter connected to the negativebus 38. Its base is connected through a resistor 65 with a circuit point48, so that during each positive needle pulse 16 the transistor 64 ismade briefly fully conductive and discharges the first capacitor 62 to agreat extent, so that the potential at the inverting input 53 of theoperational amplifier then corresponds approximately to the potential ofthe negative bus 38, and thereafter rises again while the capacitoragain charges up through the charging resistor 63. This is shown inlines (A) and (B) of FIG. 2.

The positive needle pulses are there also designated 16. They produce ineach case at 66 a discharge of the first capacitor 62. The chargevoltage u_(C) 62 is shown with reference to the line (B) of FIG. 2.

The threshold value voltage of the operational amplifier 51 is theredesignated u_(t). So long as the voltage across the first capacitor 62does not reach u_(t), the operational amplifier 51 has a positivepotential at its output, this potential being shown with reference toline (C) of FIG. 2, and designated 67. When the voltage across the firstcapacitor 62 oversteps the value u_(t), the potential at the output 54of the operational amplifier jumps to a lower value designated 68 withreference to line (C) of FIG. 2.

A setback transistor 71 is also controlled by the positive needle pulses16. The collector of the transistor 71 is connected with thenoninverting input 52 of the operational amplifier, its emitter with thenegative bus 38, and its base through a resistor 72 with a circuit point48. The setback transistor 71 is thus made conducting simultaneouslywith the discharge transistor 64 and connects the input 52 with thenegative bus 38. In consequence, at the output 54 of the operationalamplifier, there arises quite briefly the identical effect as in thecase of a high charge of the first capacitor 62 (i.e., this outputbecomes more negative and takes the potential 68). Since in thisoperation the first capacitor 62 is still involved in discharge, it ismade sure that the potential at the input 53 of the operationalamplifier thereby sinks more slowly, while the fall of potential at theinput 52 of the operational amplifier takes place suddenly. In otherwords, this setback operation takes effect in each case from the output54 onwards. This is shown in FIG. 2.

There, the needle pulses 16 for a rapidly rising speed are illustrated.In line (B) the five first sawteeth do reach the voltage u_(t), but thefollowing ones no longer do so. So long as u_(t) is reached, the effectof the setback transistor 71 at the output 54 is practically nil. Thesomewhat delayed rise at 75 shown on line (C) of FIG. 2, which isexaggerated in the drawing, is caused by the capacitor 84'. When u_(t)is no longer reached, each pulse 16 causes the setback transistor 71 tobecome conducting, and a brief needle pulse 76 appears at the output 54.These needle pulses 76 prevent the speed of the motor from risingunconstrained when the reference value is exceded and, hence, have animportant function.

Their generation by the setback transistor 71 which operates on theinput of the operational amplifier 51 has been found particularlyadvantageous because by virtue of the first capacitor 62, a veryfavorable small time delay is produced that is here used in a veryeffective way and also because the setback transistor 71 does not affectthe shape of the output signal at the output 54 (see line (C) of FIG. 2)when the speed is too small.

The further processing of the signal coming from the output 54 will nowbe described. The cathode of a diode 80 is connected to the output 54and has its anode connected through a resistor 81 with the collector ofa pnp transistor 82 and to one electrode of a second capacitor 83 whichis also connected through a resistor 84 with the output line 18 and withanother resistor 85 that connects back to the anode of the diode 80. Theother electrode of the second capacitor 83 which may, for example, havea value of 10 nF, and the emitter of the transistor 82 are connectedwith the positive voltage bus 41. The transistor 82 serves as the chargenormalizing member for the capacitor 83. This means that when thistransistor is conducting, it normalizes the charge of this capacitor atthe value zero. The base of the transistor 82 is connected via aresistor 83' with the positive voltage bus 41 and via a capacitor 84'with the output 54 of the operational amplifier 51, which output is alsoconnected through the resistor 85' with the positive voltage bus 41.

When the output 54 of the operational amplifier 51 jumps at the momentt1 from the potential 67 to the more negative potential 68, as shownwith reference to line (C) of FIG. 2, the transistor 82 is turned onthrough the capacitor 84', and while briefly conducting, it dischargesthe second capacitor 83.

Line (D) of FIG. 2 shows the potential u₈₆ at the (lower) electrode 86of the second capacitor 83. Upon discharge, this potential jumps to thepotential of positive bus 41 which is designated U₄₁ with reference toline (D) of FIG. 2. The charge voltage of the capacitor 83 is designatedu_(C83) with reference to line (D) of FIG. 2. The voltage between theoutput line 18 and the negative voltage bus 38 is there designated u₁₈.As can be seen from this portion of FIG. 2, u₁₈ and u_(C83) when added,form a value that corresponds to the output voltage of the voltageregulator 39, thus, for example, five volts. The voltage u₁₈ is thenused for further processing in the regulating amplifier or "controller"19.

After the discharge at the moment t₁, the second capacitor 83 is chargedthrough the diode 80 and the resistor 81 for a period extending throughmoment t₂, at which time the potential at the output 54 of theoperational amplifier 51 again jumps to a positive value. The value ofthe voltage u_(C83), which the second capacitor 83 reaches, is thus adirect measure for the spacing and time between the moments t₁ and t₂,and this measure is designated Δt in FIG. 2C. The value Δt is large atlow speed and accordingly u_(C83) is then large and u₁₈ is small. Withincreasing speed, Δt become smaller and smaller, so that u_(C83)likewise becomes ever smaller and u₁₈ becomes ever greater, until themaximum value of u₁₈, namely, u₄₁, is reached.

FIG. 5 shows this dependence on the frequency graphically: withincreasing frequency, the value of u₁₈ rises monotonically from aboutzero to U₄₁ in a very small frequency region of a few hertz, typically1/50 to 1/200 of the frequency at which the speed is regulated. If thedesired speed is exceeded, because the motor has very suddenly speededup, the negative needle pulses 76 (line (C) of FIG. 2) of the output 54of the operational amplifier 51 cause the second capacitor 83 to benewly discharged through the charge normalizing component 82 at everyneedle pulse (i.e., no false value can remain stored in this capacitor). If these needle pulses 76 were not produced, the value of voltageacross the capacitor 83 designaed 90 with reference to line (D) of FIG.2, would remain stored and would falsely simulate a speed that is toolow. This is this thus prevented by the needle pulses 76 in a highlyreliable manner and with a minimum of components and expense.

The signal u₁₈ is supplied to the controller 19 which contains anoperational amplifier 92 of which the output 93 is connected to a P-Ifeedback branch (series connection of the resistor 94 and the capacitor95) to the inverting input 96 of the operational amplifier. A capacitor97 is provided in parallel with P-I feedback network 94,95 forfiltering. The noninverting input 98 is connected via a resistor 99 withthe tap of a voltage divider composed of two resistors 101 and 102 ofabout the same value connected between the positive and negative busses41 and 38.

Two diodes 103 are provided between the inputs 96 and 98 inanti-parallel connection and thereby limit the output signal of thecontroller 19 and thus also the current in the motor 12.

As can be seen from FIG. 5, at the desired speed the voltage u₁₈ lies atabout half the value of U₄₁ (i.e., the two inputs of the operationalamplifier 92 then have about the same potential). Above the desiredspeed u₁₈ is so high that the operational amplifier blocks and the motor12 receives no current. For a speed that is too low, u₁₈ is too small,the signal at the output 20 becomes large and the current in the motor12 is regulated at the maximum permissible value.

The current regulator 21 which is operated by the output of thecontroller 19 contains an operational amplifier 105 that controls apower transistor 106 in the current circuit of the motor 12. A measuringresistance 107 (0.1 ohm) serves to measure the current by producing avoltage which is supplied via a resistor 108 to the noninverting input109 of the amplifier 105, which is connected through a resistor 110 tothe tap 111 of a potentiometer 112 that is fed with the output voltageof the voltage regulator 39 and serves to fit or match the currentregulator 21 to the preceeding controller 19. The output 93 of theoperational amplifier 92 is connected through a resistor 113 with theinverting input of the operational amplifier 105, where a resistor 114is also connected leading to the negative voltage bus 38.

Thus when the controller 19 provides a large signal, the current in themotor 12 is regulated at a high value. If the speed increases, thecurrent in the motor diminishes. When the speed rises above the desiredvalue, the current in the motor 12 is brought down effectively to zero,thanks to the safety feature provided by the setback transistor 71, sothat a run-up to excessively high speeds is safely avoided.

FIGS. 3 and 4 refer to a second embodiment of the invention. The portionshown in FIG. 3 corresponds to the frequency-to-voltage converter 17 ofFIG. 1. The other portions are not shown again here because these areunchanged. Parts that are the same or operate in the same way as thoseas FIG. 1 are designated with the same reference numerals as there andgenerally not further described.

The first capacitor 62, its charging resistor 63, its dischargecomponent 64 and the differentating network 46, 57 are the same as inFIG. 1. The tap 58 of the voltage divider 56, 57 is in this caseconnected directly to the noninverting inputs 52 of the operationalamplifier.

An npn setback transistor 120 is here connected through a resistor 121to the circuit point 48 so that with every positive needle pulse 16 itbecomes conductive. Its collector is connected to the output 54 of theoperational amplifier and its emitter to the negative bus 38, so thatit, when it conducts, puts the output 54 of the operational amplifiersubstantially at the potential of the negative bus 38.

The second capacitor 123 here has one of its electrodes connected to thenegative bus 38 and is charged through a constant current circuit 124 ata current I derived from the positive voltage bus 41 so long as theoutput 54 of the operational amplifier has a low potential. Anadjustable resistor 125 serves for adjustment of this constant current.Turning on the constant current is performed through a resistor 126connected to the output 54 of the operational amplifier.

A charge normalizing member in the form of an npn transistor 127 isprovided in order to discharge the second capacitor 123 every time whenthe potential at the output 54 of the operational amplifier jumps to alower value. For this purpose, the output 54 is connected via acapacitor 128 with the base of a pnp transistor 129 that is connectedthrough a resistor 130 with the positive voltage bus 41, to which theemitter of the transistor 129 is also connected. The collector of thattransistor is connected through a resistor 131 with the base of thetransistor 127, which on its part is connected through a resistor 132with the negative voltage bus 38.

FIG. 4 is referred to for explanation of the circuit of FIG. 3. Thelines (A) and (B) correspond with the lines (A) and (B) of FIG. 2 sothat reference to the latter can be made. When the voltage of the firstcapacitor 62 oversteps the value u_(t), the output 54 becomes suddenlymore negative and takes a potential 135 as illustrated at t₃ in line (C)of FIG. 4.

When the first capacitor 62 is then discharged (moment t₄) via thetransistor 64, the output signal at the output 54 becomes positive againand takes a potential 136.

The potential jump at the moment t₃ makes the transistor 129 and throughit also the transistor 127 conducting for a brief period and dischargesthe second capacitor 129, as shown at 137 in line (D) of FIG. 4.Thereupon, the second capacitor 123 is charged through the constantcurrent circuit 124 with the constant current I until the moment t₄. Atthe moment t₄, the constant current circuit 124 shuts off because thepotential at the output 54 of the operational amplifier again takes thevalue 136, and the capacitor 123 holds the voltage u_(C123) thus reacheduntil this capacitor is again discharged as shown at 137 with referenceto line (D) of FIG. 4.

The circuit of FIG. 3 in itself operates very well, but as the result ofdifferent signal transit times a so-called glitch can be produced thatdisturbs the regulation. Thus, as explained, the transistor 120 isbriefly turned on by the pulses 16 and thereby lowers the potential atthe output 54. When this event occurs at the same time as the trailingedge of the signal u₅₄ (see line (C) of FIG. 4) there is no disturbance,but if this event occurs shortly before or shortly thereafter, it canmake trouble. Line (C) of FIG. 4 shows at 140 such an undesiredsupplemental pulse that is produced by the transistor 120 being turnedon. This pulse 140 has the effect, through the capacitor 128, that thetransistors 129 and 127 become briefly conducting and provide anuntimely discharge of the second capacitor 123, as shown at 141 withreference to line (D) of FIG. 4. This misleads the control circuit bysimulating an excessively high speedp i.e., a false regulation processresults. Such disturbing pulses in practice do not appear in circuitsthat are built up of discrete components, but they can appear in thecase of integrated circuits. In other words, the circuit of FIG. 1 ismore suitable for use in or with more integrated circuits.

As can be observed with reference to line (D) of FIG. 4, the averagevoltage u_(C123) of the second capacitor 123 sinks with increasing motorspeed. For this reason the connections 96 and 98 must be interchanged atthe controller 19 (FIG. 1) in order to obtain its correct controllercharacteristics when using the circuit of FIG. 3. In other respects thecontroller 19 and the current regulator 21 can be constituted in thesame manner as in FIG. 1.

When the desired speed is exceeded, the voltage at the first capacitor62 remains constantly below the value u_(t) and the operationalamplifier 51 does therefore not alter its output signal any more, whichmeans that the last stored value at the second capacitor 123 (142 inFIG. 4) is continuously maintained. However, since the transistor 120 isbriefly conducting at the occurence of each needle pulse 16, briefnegative needle pulses 150 are produced at the output 54 in this casealso, which activate the transistor 127 and discharge the secondcapacitor 123. The charge value stored in the second capacitor 123 thenremains at zero until the motor speed again falls below the desiredvalue. Here also, no false value can remain stored in the secondcapacitor 123, and the voltage value of this capacitor is continuouslyupdated.

FIG. 6 shows a third embodiment of the invention in the form of avariant of FIG. 1. The same or identically operating parts as in FIG. 1are given the same reference numerals in FIG. 6 and in general are notnow further described. This variant concerns only the frequency tovoltage converter 17 of FIG. 1. The other portions of the circuit areidentical with FIG. 1. The modified frequency-to-voltage converter isdesignated 152 in FIG. 6. The transistor 71 of FIG. 1 and its associatedcomponents are completely dispensed with in FIG. 6. The circuit of thedischarge transistor 64 is substantially like that in FIG. 1. Theresistor 47 lies directly between the base and emitter of the transistor64. Between the output 30 of the operational amplifier 29 and the baseof the transistor 64 is located a series-connected combination of thecapacitor 46 and the resistor 65. The capacitor 46 and the resistors 65and 47 function as a differentiating network as in FIG. 1, so that thetransistor 64 is briefly made conducting by the positive flank of eachof the signals 31, discharging the first capacitor 62 as has beendescribed in detail with reference to FIG. 1. Here also, the operationalamplifier 51 operates as a comparator (i.e., the potential of its output54 becomes suddenly more negative when the potential at its input 53exceeds that at its input 52, as has also been described fully withreference to FIG. 1) so that then--in a negative potential jump--thecapacitor 83 is discharged through the transistor 82, after which thecapacitor 83 is charged through the resistor 81 and the diode 80 so longas the output 54 remains negative so that its charge is a measure forthe actual speed. This charge voltage at the second capacitor 83 is thensupplied to the controller 19 over the line 18 in the manner describedin detail with reference to FIG. 1. When the speed becomes too high, thepotential of the output 54 would remain continously positive because thefirst capacitor 62 no longer reacher the necessary voltage. A chargestored at the second capacitor 83 could then be held steady and thecircuit would then continuously be misled by what appear to be too low aspeed as has already been explained in connection with FIGS. 1 and 2(see reference numeral 90).

In order to avoid this difficulty, a second differentating network isconnected in this case to the output 30, for this purpose between thisoutput and the inverting input 53 a series combination of a smallcapacitor 153 (e.g., 22 pF) and a resistor 154 (e.g., 2.2 k ohms).

Furthermore, a relatively high-resistance resistor 155 (e.g., 47 k ohms)is provided for decoupling (compare the resistor 59 of FIG. 1) betweenthe inverting input 53 of the operational amplifier and the collector ofthe transistor 64.

If now the potential at the output 30 of the operational amplifier 29jumps to a more positive value, the transistor 64 is turned on anddischarges the first capacitor 62, an operation that lasts for a certaintime. At the same time the differentiating network consisting of thecapacitor 153 and the resistor 154 supplies a brief positive needlepulse 156 to the inverting input 53 of the operational amplifier 52, asthe result of which a negative needle pulse appears quite briefly at theoutput 54 of the comparator 51.

This negative needle pulse briefly turns on the pnp transistor 82 andthereby produces a discharge of the second capacitor 83, in this casealso, which is to say when the speed is too high, so that the controller19 is not misled by an apparently low speed, but instead the capacitor83 actually takes on a voltage that corresponds to an excessively highspeed. On the other hand, this negative needle pulse is so short thatthe capacitor 83 cannot charge up over the resistor 81 within the shortduration of the pulse.

The relatively large resistance 155 prevents the positive needle pulses156 from being short-circuited through the first capacitor 62 and thusdecouples the signal sources from each other.

In the variant circuit illustrated in FIG. 6, the differentiated needlepulses 156 are not amplified but are supplied directly to the input 53.In this manner the pulses have a minimum transit time, and no disturbingeffects can arise such as were described in connection with FIGS. 3 and4 with reference to the designations 140 and 141 because in that case,relatively long lifetimes for these pulses were involved. Themodification illustrated in FIG. 6 is hence unusually simple and,furthermore--on account of the extreme small signal transit times--mostreliable and certain in operation.

If necessary, the capacitor 153 can be omitted in the variant circuit ofFIG. 6 and instead the resistor 154 can be connected between thecapacitor 46 and the resistor 65, as is indicated by the broken line157. One capacitor is saved thereby, which can be of advantage in thecase of an integrated circuit. The capacitor 46 (e.g., 220 pF) is thencommon to the two differentiating networks.

Although the invention has been described with reference to particularillustrative examples, it will be understood that further variations andmodifications are possible within the inventive concept.

We claim:
 1. Speed control apparatus for a rotary machine comprisingsignal generating means for producing a signal of a frequencyproportional to rotary speed and having also torque varying means foradjusting the rotary speed of the machine:(a) a first capacitor (62), acharging circuit (63) and a discharging circuit (64) therefor, saiddischarging circuit being constituted so as to discharge said firstcapacitor in a short period of time periodically in response to signals(16) derived from the signal (14) produced by said signal generatingmeans; (b) a threshold circuit (51) to which said first capacitor isconnected for providing an output signal when the voltage across saidfirst capacitor exceeds a predetermined value; (c) a second capacitor(83,123) connected so that its charge condition is controlled by saidoutput signal of said threshold circuit; (d) means (19,20) responsive tothe voltage across said second capacitor for actuating said torquevarying means of said rotary machine; (e) active normalizing circuitmeans (82;127) connected to said second capacitor; and (f) means (71;120; 153-155) for activating said active normalizing circuit meansbefore each new changes of the charge condition of said second capacitorproduced by said threshold circuit output signal and thereby bringingthe charge condition of said second capacitor substantially to apredetermined charge value before the beginning of each new charging ofsaid second capacitor by said threshold circuit output signal.
 2. Speedcontrol apparatus as defined in claim 1, in which said predeterminedcharge value of said second capacitor is substantially zero chargevalue.
 3. Speed control apparatus as defined in claim 1, in which adifferentiating network (83'; 84'; 128, 130) is interposed between theoutput of said threshold circuit and said active normalizing circuitmeans (82; 127), connected for activating the latter promptly upon theappearance of an output signal of said threshold circuit responsive to arise of the voltage of said first capacitor past said predeterminedvoltage or of a signal substantially equivalent to the onset of outputsignals responsive to said rise of voltage of said first capacitor. 4.Speed control apparatus as defined in claim 3, in which said thresholdcircuit (54) is constituted as a comparator having at one input thereof(52 or 53) a connection for receiving a control pulse simultaneouslywith the activation of said discharging circuit (64) of said firstcapacitor (62) whereby there is produced briefly at the output of saidcomparator a signal substantially equivalent to the onset of an outputsignal of said comparator which is responsive to a rise of voltage ofsaid capacitor past said predetermined voltage.
 5. Speed controlapparatus as defined in claim 4, in which a second differentatingnetwork (153,154) is connected to an input (53) of said comparatorthreshold circuit (51) for providing thereto a brief sharp pulsesynchronously with the onset of discharging of said first capacitorthrough its discharging circuit.
 6. Speed control apparatus as definedin claim 4, in which amplifying means (71) are provided having theoutput thereof connected to an input (52) of said comparator thresholdcircuit (51) for furnishing the brief activating pulse thereto in thecorrect sense of producing a corresponding brief output pulse, saidamplifying means having its input connected so as to provide said pulseat its output synchronously with the onset of discharging of said firstcapacitor (62) through its discharging circuit (64).
 7. Speed controlapparatus as defined in claim 5 or claim 6 in which a relatively highresistance (59; 155) is connected between said input of said comparatorthreshold circuit (51) to which a brief pulse is applied synchronouslywith the onset of discharging of said first capacitor, so as to decouplethe application of said pulse to said input from another signal appliedto said input.
 8. Speed control apparatus as defined in claim 1, 2 or 3in which electronic switching means (120) are provided responsive to thesame signals (16) to which said discharging circuit (64) of said firstcapacitor is responsive and are connected for producing, at the output(54) of said threshold circuit (51), a signal (150) substantiallyequivalent to the onset of an output signal of said threshold circuitresponsive to the rise of the voltage across said first capacitor pastsaid predetermined value of voltage thereof.
 9. Speed control apparatusaccording to claim 1 wherein said active normalizing circuit means haveat least one parallel passive circuit means.
 10. Speed control apparatusaccording to claim 9, said passive circuit means is a resistor.